How to determine Specular Weight?

I'm trying to understand how to determine the specular weight of common materials. I have a jacket material, which at a spec weight of 1.0, completely washed out my color as a result of the spec reflection with a high roughness value. I've finally determined that I had to turn down the IOR and that has my color feeling right at a spec weight of 1. But i'm left with a lot of questions about the spec weight parameter. Is spec weight one of those things that should just be modulated to taste / or what looks right? or are there specific values for specfic materials to keep things physically accurate? It seems like when you're working in substance with metal / rough - there is no equivalent spec weight parameter. It's advised to plug in the roughness map to spec roughness and keep spec weight at 1. But how can that be? How do all these materials have the same specular weight? Is it that the roughness map modulates both the glossness / smoothness / clarity of the specular reflection as well as the intensity? I'm not sure how that could be, though, since, when you up the value of the spec roughness, with no map, it doesn't modulate the weight.

I fully understand the difference between specular weight and specular roughness in theory, and I've read the arnold documentation, I'm just still unsure of when to change the values of specular weight in practice? Or how do you know when a material has a different spec weight than another? Thanks so much in advance for any information you can provide!

1 Reply

It depends on how you painted your maps. The specular weighting is just a multiplier on the specular colour. So if you already painted accurate reflection values in your specular colour map, it can remain at one.

Looking up reflectance values is often tricky. Most of the materials we deal with/recreate are impure, so the pure material data will not match anyway. But a good starting point is:

Dielectrics: 0.02-20 %

Conductors: 40%-95%

There aren’t many natural materials out there in the 20-40% grey zone. 20% is really at the high end of the dielectrics spectrum, for example diamond.